* PCBC-527: Initial tracing implementation. The extension checks if libcouchbase has tracing support, and sets up hooks for fetch/persist operations to report encoding/decoding times. This functionality is preview and is not enabled by default.
* PCBC-519: Implement log redaction. When `log_redaction=on` is specified in the connection string, the library will wrap sensitive data in the logs in special tags, which can be processed by the `cblogredaction` tool from the server distribution.

Changes:
* PCBC-331: CouchbaseBucketManager now has a flush operation to clear a bucket.
* Orphan: Minor fix to view management return values.
* Orphan: ViewQuery::group_level now can be chained as expected.
* PCBC-334: PHP 5.4 transcoder compatibility issues have been fixed.
* PHP 5.3 support has been deprecated and PHP 5.6 is now fully supported.
* Corrected various memory leaks and segfaults.